Frozen water on Mercury reveals its cooler side

NASA’s Messenger spacecraft has found that water ice likely exists on the frozen poles of Mercury, the planet closest to the sun.

Messenger, which has been orbiting Mercury since 2011, appears to confirm a long-held theory by astronomers: that the parts of Mercury’s poles which never receive sunlight contain frozen water.

Using a neutron spectroscopy, Messenger measured average hydrogen concentrations, which suggested that there was water located at the polar regions. The data implies that while the coldest parts of Mercury’s poles may contain ice at their surface, much of it appears to be trapped under an identified dark material in areas that are too warm for ice to be stable at the surface.

Sean Solomon, principal investigator of the Messenger mission says: "For more than 20 years the jury has been deliberating on whether the planet closest to the Sun hosts abundant water ice in its permanently shadowed polar regions. Messenger has now supplied a unanimous affirmative verdict."

However, the dark material on the pole’s surface introduces a new set of mysteries for the scientists investigating Mercury.

"The new observations have also raised new questions," says Solomon. "Do the dark materials in the polar deposits consist mostly of organic compounds? What kind of chemical reactions has that material experienced? Are there any regions on or within Mercury that might have both liquid water and organic compounds? Only with the continued exploration of Mercury can we hope to make progress on these new questions."

To analyse the surface of the poles, Messenger used a piece of equipment called the Mercury Laser Altimeter (MLA), which fired more than 10 million laser pulses at Mercury to create a detailed map of the planet’s surface, including craters that were likely to be among its coldest regions. MLA measurements of the planet’s shadowed north pole revealed regions of irregular dark and light deposits which the astronomer’s believe are the areas of frozen water ice. The findings are published in a series of papers in the journal Science.